This invention relates to a surface acoustic wave device using a lithium tetraborate (Li.sub.2 B.sub.4 O.sub.7) single crystal which is a piezoelectric material.
A surface acoustic wave device is known which is adapted to propagate a surface acoustic wave on a piezoelectric substrate on which an aluminum electrode is formed. The temperature coefficient of the delay time (hereinafter referred to merely as a TCD) and electromechanical coupling coefficient K.sup.2 (hereinafter referred to merely as a coupling coefficient K.sup.2) are important to a piezoelectric substrate. Here, the delay time means the time required for the surface acoustic wave to be propagated between two points, and the ratio in which it varies in accordance with the temperature is called a "TCD". TCD is preferred to have a smaller absolute value below 10 ppm/.degree.C. for use on a resonant element. The coupling coefficient K.sup.2 is the conversion efficiency of electric energy to surface acoustic wave energy, and the greater the coupling coefficient the better.
FIG. 1 shows the characteristics of a typical conventional substrate material (single crystal) for a surface acoustic wave. FIG. 1 shows a plot of the various materials with TCD (ppm/.degree.C.) as the abscissa and the coupling coefficient K.sup.2 (%) as the coordinate. An X-cut 112.degree. Y-propagating X-112.degree.Y lithium tantalate (LiTaO.sub.3) has a coupling coefficient K.sup.2 of a moderate value of about 0.8%, but its TCD is greater value of about 20 ppm/.degree.C. On the other hand, an ST-cut (42.degree.Y-X) quartz has a TCD of 0, but its coupling coefficient K.sup.2 is a very small value of 0.1%. A 128.degree.-rotation Y-cut X-propagating (128.degree.Y-X) lithium niobate LiNbO.sub.3 has a coupling coefficient K.sup.2 of 4.8%, a value greater than that of LiTaO.sub.3, but its TCD is a greater value of about 80 ppm/.degree.C. which is unpractical. From this it will be appreciated that it is desirable to have a coupling coefficient K.sup.2 of 0.8 to 1.0% as in the case of LiTaO.sub.3 and a TCD of about 1/5 as in the case of LiTaO.sub.3, preferably 0. Recently, attention has been paid to Li.sub.2 B.sub.4 O.sub.7 as a desirable substrate for the surface acoustic wave. FIG. 1 shows the characteristics of a Li.sub.2 B.sub.4 O.sub.7 single crystal (20.degree.X-Z) which has been obtained from tests conducted by the inventors. Li.sub.2 B.sub.4 O.sub.7 has a coupling coefficient K.sup.2 of about 1%, an improvement over that of LiTaO.sub.3, and a TCD of 0. "20.degree.X-Z" indicates that the cut surface of the substrate is perpendicular to a rotated X-axis obtained by rotating the X-axis through 20.degree. about the Z-axis toward the Y-axis, and that the propagation or transfer direction of the surface acoustic wave is parallel to the Z-axis. "20.degree.X-Z" may be transformed to Eulerian angles (110.degree., 90.degree., 90.degree.). Here, TCD is found through the calculation of the measured value of the oscillation frequency of an oscillator circuit using a surface acoustic wave delay line in which input and output electrodes comprised of normal interdigital electrodes are formed on the surface of the Li.sub.2 B.sub.4 O.sub.7 single crystal substrate.
The surface acoustic wave device is also widely accepted for a surface acoustic wave resonator and surface acoustic filter. The surface acoustic wave resonator is implemented by forming terminal electrodes, comprised of interdigital electrodes, on the middle portion and grating reflectors one at each side portion of a piezoelectric substrate, noting that all of them are formed of evaporated films made principally of aluminum. The surface acoustic wave filter is realized by forming input and output electrodes, made up of interdigital electrodes, on a piezoelectric substrate, noting that these electrodes are formed of evaporated films made principally of aluminum. Since the energy of the surface acoustic wave is concentrated toward the surface of the substrate, the propagation characteristic of the surface acoustic wave is largely influenced by the surface conditions of the substrate. It has been found by the tests undertaken by the inventors that the temperature characteristic (TCD) deteriorates due to the formation of the Al-evaporated films. That is, the Li.sub.2 B.sub.4 O.sub.7 single crystal (20.degree.X-Z) substrate, though having a better TCD of its own, does not have as good a frequency/temperature characteristic, after the evaporated films, such as interdigital electrodes, principally made of aluminium have been formed.
In order to consider the deterioration of the characteristics due to the forming of the Al-evaporated films, the inventors constructed an oscillator circuit using a delay line with a dummy electrode of an Al-evaporated film uniformly formed, on a propagation path for a surface acoustic wave, between input and output electrodes. An optimum cut angle, at which the substrate is cut from an Li.sub.2 B.sub.4 O.sub.7 single crystal to obtain a better TCD, and the direction of propagation of the surface acoustic wave, were found through the measurement of the oscillation frequency. This technique was filed under U.S. patent application Ser. No. 508,044 on June 27, 1983 entitled "Surface Acoustic Wave Device", and under European patent application No. 83303734.4 on June 28, 1983, entitled "Surface Acoustic Wave Device". The surface acoustic device here tested is for one type of the surface acoustic wave delay line and the temperature dependency of the delay time between input and output transducers is improved, but not the temperature dependency of the resonance frequency for the surface acoustic wave resonator or of the center frequency for the surface acoustic wave filter. Further experiments prove that a better temperature characteristic is not obtained for the surface acoustic wave device in which the interdigital terminal electrodes and/or grating reflectors are formed on a piezoelectric substrate having a determined cut angle and propagation direction.
Furthermore, since the Li.sub.2 B.sub.4 O.sub.7 single crystal has an anisotropy as in the case of the other single crystal material, a slight change in the cut angle at which the substrate is cut from the crystal causes a greater variation, for example, in TCD and in the propagation speed of the surface acoustic wave, within a certain range. A resonant frequency f.sub.r (a central frequency f.sub.o) is expressed by v/2l. Here, v is a propagation velocity of the surface acoustic wave and l is a pitch of an interdigital electrode and a grating reflector. It is therefore difficult to manufacture surface acoustic devices of the same characteristics with better reproducibility, without involving a change in the temperature characteristic, resonant frequency, center frequency, and so on.